Narrowband Internet of Things (NB-IoT) is an emerging cellular technology that will provide improved coverage for massive number of low-throughput low-cost devices with low device power consumption in delay-tolerant applications. A new single tone signal with frequency hopping has been designed for NB-IoT Physical Random Access Channel (NPRACH).
The Networked Society and IoT are associated with new requirements on cellular networks with respect to device cost, battery lifetime, and coverage for example. To drive down device and module cost, using a system-on-a-chip (SoC) solution with integrated power amplifier (PA) is highly desirable. However, it is feasible for the current state-of-the-art PA technology to allow 20-23 dBm transmit power when the PA is integrated in a SoC. This constraint limits uplink “coverage”, which is related to how much path loss is allowed between a user terminal and a base station. To maximize the coverage achievable by an integrated PA, it is necessary to reduce PA backoff. PA backoff is needed when the communication signal has non-unity, peak-to-average power ratio (PAPR). The higher the PAPR, the higher PA backoff required. Higher PA backoff also gives rise to lower PA efficiency, and thus lower device battery lifetime. Thus, for wireless IoT technologies, designing an uplink communication signal that has as low PAPR as possible is critically important for achieving the performance objectives concerning device cost, battery lifetime, and coverage.
Currently 3GPP is standardizing NB-IoT technologies. There is strong support from the existing Long Term Evolution (LTE) eco-system (vendors and operators) for evolving existing LTE specifications to include the desired NB IoT features. This is motivated by the time-to-market consideration, since a LTE-based NB-IoT solution can be standardized and developed in a shorter time frame. LTE uplink however is based on single-carrier frequency-division multiple-access (SC-FDMA) modulation for the uplink data and control channels, and Zadoff-Chu signal for random access. Neither of these signals has good PAPR properties.
To help solve the above mentioned problems, a new random access preamble signal has been proposed and adopted in 3GPP. The new random access preamble signal is referred to as a single-tone frequency hopping NB-IoT PRACH (NPRACH). (See RAN1 Chairman's Notes, 3GPP TSG RAN WG1 Meeting #84 St Julian's, Malta, 15-19 Feb. 2016.) Generation of the random access preamble signal includes concatenating N preamble symbol groups, each preamble symbol comprising a single tone. NPRACH uses the following hopping pattern, resulting in at least four different subcarrier frequencies. Inner layer fixed-size hopping is applied within every four symbol groups. First level single-subcarrier hopping is used between the first and the second symbol groups and between the third and the fourth symbol groups. Further, the two single-subcarrier hoppings are mirrored, i.e., if the first hopping is “UP”, the second hopping is “DOWN”, and vice versa. Second level 6-subcarrier hopping is used between the second and the third symbol groups. Outer-layer pseudo-random hopping is applied between groups of four symbol groups.
The new NPRACH signal is single-tone based and has extremely low PAPR, and thus reduces the need for PA backoff to the greatest extent and maximizes PA efficiency. The new NPRACH signal is compatible with SC-FDMA and orthogonal frequency-division multiple-access (OFDMA) as in any OFDM symbol interval, the new NPRACH signal looks like an OFDM signal of one single subcarrier.
In one embodiment, a radio network node such as a base station receives the NPRACH signal from a wireless communication device, and a baseband processor within the radio network node processes the received signal in an attempt to detect a random access preamble that comprises multiple symbol groups, with each of the symbol groups on a single tone during a different time resource, according to a frequency hopping pattern that hops the single tone different frequency distances at different symbol groups, wherein each symbol group comprises one or more symbols.
Note that for a single subcarrier signal, the OFDM signal is identical to the SC-FDMA signal. Further, hopping patterns are carefully designed such that (1) accurate time-of-arrival estimation can be performed by the base station, (2) the frequency resources can be fully utilized by PRACH while maintaining orthogonality of different preambles.